The invention relates to a glass ceramic that is distinguished by a high thermal and chemical stability and which is suitable, in particular, as a substrate for coating with semiconductor materials such as GaAs, or as a substrate for optical components.
Substrate materials used in coating processes must conform to exacting requirements. The surface of the substrate must meet the highest requirements in respect of roughness, because any existing unevenness can be transferred to any coatings applied. The thermal expansion of the substrate should be adapted to the applied coating in order to avoid detachment upon temperature variations. Depending on the coating process, high thermal stability to at least 800° C. or more is essential. Transparency of the substrate is also desired, depending on the specific application. In many cases, highly transparent substrates make it easier to perform quality checks, for example. A certain amount of light scattering in the substrate may also be required. This is the case in the photovoltaics field, for example, in that the light path in a solar cell can be extended by a specially applied light-scattering layer. To enable perfect cleaning of the substrate, a sufficient chemical resistance against commonly used cleaning agents is also necessary. Furthermore, the substrate materials should be inexpensive and producible in dimensions that can be chosen as far as possible at will. Conventional substrate materials often fail to meet the above requirements to an adequate extent. The use of glass is frequently a non-option due to the necessary deployment at high temperatures, whereas ceramics have disadvantages in respect of surface and forming characteristics. Monocrystals, which mostly provide an ideal solution from the technical perspective, are generally expensive to produce and available only in limited sizes.
Glass ceramics are an appropriate alternative due to their high thermal stability compared to glass, the diversity of molding options known from glass production as well as the possibility of producing them to be transparent.
A glass ceramic is known from US 2005/0096208 A1 that contains 35 to 60 wt.-% SiO2, >4 to 10 wt.-% B2O3, 0 to 10 wt.-% P2O5, 16.5 to 40 wt.-% Al2O3, 1 to 10 wt.-% TiO2, 0 to 8 wt.-% Ta2O5, 0 to 6 wt.-% Y2O3, 1 to 10 wt.-% ZrO2, 6 to 20 wt.-% MgO, 0 to 10 wt.-% CaO, 0 to 4 wt.-% SrO, 0 to 8 wt.-% BaO, 0 to 4 wt.-% ZnO, wherein the total content of SnO2 and CeO2 amounts 0 to 4 wt.-%, wherein the total content of SO42− and Cl− amounts to 0 to 4 wt.-%, and wherein the total content of SnO2, CeO2, SO42− and Cl− is between 0.01 and 4 wt.-%. This glass ceramic is suitable, in particular, for production by the float glass method. It has a coefficient of thermal expansion in the range between 4×10−6/K and is therefore suitable, for example, as a substrate material for silicon (thermal expansion coefficient of 3.7×10−6/K).
However, many technically relevant semiconductors, such as GaAs or other III/V semiconductors, for example, have a significantly higher coefficient of thermal expansion in a range of 5 to 8×10−6/K. Thus, the known substrate is not especially suitable for such semiconductors.
In the case of substrates to be used in the field of optical applications, optical transparency and other optical properties are required in addition to the characteristics described above, including, in particular, the optical position in the Abbé diagram and the refractive index.
A glass ceramic consisting of 10 to 50 wt.-% SiO2, 5 to 35 wt.-% B2O3, 25 to 75 wt.-% P2O5, wherein the total content of SiO2, P2O5 and B2O3 is greater than 90 wt.-%, is known from U.S. Pat. No. 4,576,920. This glass ceramic may also contain up to 10 wt.-% of at least one oxide of the group of Li2O, Na2O, MgO, CaO, SrO, BaO, CdO, ZnO and SnO2, wherein the amount of any single constituent may not exceed 5%. The glass ceramic has BPO4 as its main crystal phase. Its thermal stability is specified as ranging to about 1100° C., with a thermal coefficient of expansion ranging between 4.5 and 6.5×10−6/K.
However, it has been found that one disadvantage of this glass ceramic is that it does not have sufficient chemical resistance against acids and/or alkalis. This property is incompatible with use of the glass ceramic in practice, because cleaning processes would attack the surface of the glass ceramic. One way of solving this problem is to increase the amount of silicon in the glass ceramic. However, this option is limited by the meltability of the starting glass at economically interesting temperatures (<1650° C.), which requires a percentage of silicon oxide that is generally less than 50%.
One material having BPO4 as its main crystal phase and containing 50 to 65 wt.-% SiO2 is known from U.S. Pat. No. 4,833,104, but forming is done in this case by sintering due to the higher melting temperature. However, this in turn leads to disadvantages in respect of the forming, transparency and surface quality of the product.
Another sintered material containing BPO4 as its main crystal component is know from JP 04-160054, but because forming is done by powder technology, the material suffers from the same drawbacks as described in the foregoing.